In the conventional manufacture of paper sheets for use in tissue, toweling and sanitary products, it is customary to perform, prior to drying, one or more overall pressing operations on the entire surface of the paper web as laid down on the Fourdrinier wire or other forming surface. Conventionally, these overall pressing operations involve subjecting a moist paper web supported on a papermaking felt to pressure developed by opposed mechanical members, for example, rolls. Pressing generally accomplishes the triple function of mechanical water expulsion, web surface smoothing and tensile strength development. In most prior art processes, the pressure is applied continuously and uniformly across the entire surface of the felt. Accompanying the increase in tensile strength in such prior art papermaking processes, however, is an increase in stiffness and overall density.
Furthermore, the softness of such conventionally formed, pressed and dried paper webs is reduced not only because their stiffness is increased as a result of increased interfiber hydrogen bonding, but also because their compressibility is decreased as a result of their increased density. Creping has long been employed to produce an action in the paper web which disrupts and breaks many of the interfiber bonds already formed in the web. Chemical treatment of the papermaking fibers to reduce their interfiber bonding capacity has also been employed in prior art papermaking techniques.
A significant advance in producing lower density paper sheets is disclosed in U.S. Pat. No. 3,301,746 which issued to Sanford et al. on Jan. 31, 1967, said patent being hereby incorporated herein by reference. The aforesaid patent discloses a method of making bulky paper sheets by thermally predrying a web to a predetermined fiber consistency while supported on a drying/imprinting fabric and impressing the fabric knuckle pattern in the web prior to final drying. The web is preferably subjected to creping on the dryer drum to produce a paper sheet having a desirable combination of softness, bulk, and absorbency characteristics.
Other papermaking processes which avoid compaction of the entire surface of the web, at least until the web has been thermally predried, are disclosed in U.S. Pat. No. 3,812,000 issued to Salvucci, Jr. et al. on May 21, 1974; U.S. Pat. No. 3,821,068 issued to Shaw on June 28, 1974; U.S. Pat. No. 3,629,056 issued to Forrest on Dec. 21, 1971; and U.S. Pat. No. 3,994,771 issued to Morgan, Jr. et al. on Nov. 30, 1976, the aforesaid patents being hereby incorporated herein by reference.
Twin wire formation style papermachines, which are known to be old in the art, may of course be employed with the low-density papermaking processes generally described in the aforementioned patents. When utilized in conjunction with a process such as that described in the patent to Sanford et al., the prior art practice has been to form a moist fibrous web by depositing a fibrous stock slurry between a pair of converging Fourdrinier wires, partially dewatering the moist fibrous web while it is constrained between the Fourdrinier forming wires, separating the uppermost Fourdrinier wire from the web which remains in contact with the lowermost Fourdrinier wire and thereafter transferring the moist fibrous web by means of fluid pressure from the lowermost Fourdrinier wire to a less dense foraminous drying/imprinting fabric while the web is at relatively low fiber consistency. The web is thereafter processed in accordance with the teachings of the patent to Sanford et al. It has been unexpectedly discovered, however, that both finished product bulk and absorptive capacity can be improved by extending the drying/imprinting fabric to the twin wire formation zone of the papermachine, thereby eliminating completely the uppermost Fourdrinier wire as well as the necessity for subsequently transferring the moist fibrous web from the lowermost Fourdrinier wire to the drying imprinting fabric.